The present invention relates broadly to aircraft instrumentation and, more particularly, to an apparatus for determining and displaying the bank angle of a moving aircraft based on satellite navigational signals.
An aircraft turns by banking which is caused by oppositely deflecting ailerons. As the aircraft turns in a banking manner, the pilot requires some visual indicator of the aircraft banking attitude to coordinate the turn, maintain the desired course and to safely operate the aircraft. The banking attitude is the aircraft bank angle or angle of bank. Bank angle indicators are common instrumentation in an aircraft and are sometimes combined with a bubble/level device to form a combined instrument known as a turn coordinator. The display associated with this device gives the pilot necessary information regarding performance of a coordinated turn. As the aircraft turns, winds can cause lateral forces to affect the aircraft turning movement, tending to slip the aircraft sideways. The rudder may be moved in correcting response to lateral forces causing the aircraft to slip. The lateral forces are displayed on the turn coordinator using the bubble-type device similar to a carpenter's level. However, it is the bank angle which actually defines the extent and rate of the turn in coordination with aircraft speed. Current bank angle indicators or turn coordinators are based on gyroscopes and use the precession of the gyroscope in combination with a correction factor to determine bank angle. The bank angle is displayed on the turn coordinator using a tilting aircraft silhouette.
Currently, satellite guidance has become widely used for navigation and promises to be a significant advance in air navigation. The current system known as NAVSTAR or global positioning system (GPS) provides complete navigational guidance anywhere in the world, twenty-four hours a day, and is highly accurate. The system was originally developed for military use and, accordingly, reports at two levels of accuracy. The first is the precision mode for military use which is accurate to within 16 meters laterally and vertically and the coarse/acquisition mode for civilian use provides three-dimensional position accurate to within 100 meters. NAVSTAR/GPS functions virtually free of all environmental limitations providing reliability at all times of the day or night, during all seasons, in all weather conditions, and free of solar disturbances. Further, NAVSTAR/GPS offers a relatively inexpensive system from the user's point of view. While the satellites themselves are enormously expensive, the satellite navigation units are well within reach of those with such a need. The navigation receiver units may be mounted with an aircraft, other vehicles, or with hand-held, computer-coordinated displays. Further, the NAVSTAR/GPS is a passive system not requiring interrogation by the user.
NAVSTAR/GPS provides twenty-four satellites in synchronous orbit about the earth. Each satellite contains four atomic clocks so that their time and position is precisely known. Therefore, a conventional triangulation fix is possible and the time of the fix is also known. One of the inherent problems is that, while the satellites can carry four atomic clocks, the ground units typically use a quartz oscillator so that the timing on the receiving end is somewhat inaccurate. To compensate for this, a navigation signal is obtained using four satellites with the fourth satellite providing a time correction. The NAVSTAR/GPS navigational signals may be used for determining a precise location above the earth or on the earth to determine longitude, latitude and altitude. As may be expected, given the known position information and an accurate clock, derivative information may be obtained from the navigational signals and applied in various ways.